Transcript inheritance - Shelton School District

PATTERNS OF INHERITANCE
CAPTER 10
Pre-Mendel’s Theories
Blending Hypothesis: when parents
with different traits have offspring,
this will always show a blending of
the traits
Spontaneous generation: Non-living
matter giving rise to living matter
Homunculus: The little man inside
the sperm
Mendel’s experiments with pea plants
Trait: a variation of a particular
character
Particulate hypothesis: Parents
pass on to their offspring separate
and distinct factors (genes) that are
responsible for inherited traits
 Genetics: the study of heredity
Mendel’s experiments started with
True-breeding plants: when selffertilized, a true-breeding plant
produces offspring identical in
appearance to itself generation
after generation
 Cross-fertilization, sperm from the pollen of one
flower fertilizes the eggs in the flower of a
different plant
the offspring of two different true-breeding
varieties are called hybrids.
Monohybrid cross: pairing in which the parent
plants differ in only one (mono) character.
Mendel’s principle of segregation
There are alternative forms of factors
(genes) called alleles.
For each character, an organism has
two alleles for the gene controlling that
character, one from each parent.
Homozygous = same alleles
Heterozygous = different alleles
Principle of segregation (cont.)
When only one of the two different alleles
in an heterozygous individual appears to
affect the trait, that allele is called the
dominant allele. The allele that does not
appear to affect the trait is called the
recessive allele
The two alleles for a character segregate
(separate) during the formation of
gametes (sex cells). Each gamete carries
only one allele of each character
(Principle of segregation)
Phenotype refers to the
observable trait (purple flowers)
Genotype refers to the
combination of alleles (PP)
Phenotypic ratio: ratio of plants
with purple flowers to those with
white flowers (3 purple : 1 white)
Genotypic ratio: ratio of possible
combinations of alleles (1 PP : 2 Pp
: 1 pp)
Probability and Punnett Squares
Probability: Chance of
Something happening
Punnett Square: Diagram
used to show the probability
of a genetic cross
Phenotype refers to the
observable trait (purple flowers)
Genotype refers to the
combination of alleles (Pp)
• Phenotypic ratio: ratio of
plants with purple flowers to
those with white flowers (3
purple : 1 white)
• Genotypic ratio: ratio of
possible combinations of alleles
(1 PP : 2 Pp : 1 pp)
How can you find out the
genotype of an individual
showing the dominant trait?
TESTCROSS
In a testcross, an individual of
unknown genotype, but
dominant phenotype is crossed
with a homozygous recessive
individual
DIHYBRID CROSS
crossing of organisms
differing in two characters
Principle of Independent Assortment
During gamete formation in an F2
cross, a particular allele for one
character can be paired with either
allele of another character
The alleles for different genes are
sorted into the gametes independently
of one another.
Not all traits are inherited
following the patterns found by
Mendel in pea plants
Intermediate or incomplete
inheritance
Multiple alleles – codominance
Polygenic inheritance
Environment - epigenetics
Intermediate or incomplete inheritance
The heterozygotes have a
phenotype that is intermediate
between the phenotypes of the
two homozygotes
Multiple alleles - codominance
For many genes several alleles exist in the
population.
Multiple alleles control the character of blood type
in humans.
There are six possible genotypes.
The alleles IA and IB exhibit codominance,
meaning that a heterozygote expresses both traits.
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Polygenic inheritance
 When two or more genes affect a single
character
 In humans, height and skin color have
polygenic inheritance
Chromosome Theory of Inheritance
Genes are located on chromosomes
Behavior of chromosomes during
meiosis and fertilization accounts for
inheritance patterns – (chromosomes
undergo segregation and
independent assortment during
meiosis)
Gene locus: location at which alleles
of a gene reside on homologous
chromosomes
Linked genes: genes that are located
in the same region of a chromosome
Genetic linkage: tendency for the
alleles on one chromosome to be
inherited together. The closer two
genes are on a chromosome, the
greater the genetic linkage
Sex-Linked Genes (any gene
located on sex chromosomes)
 http://www.youtube.com/watch?v=vP_h08cT5jw
Morgan's monohybrid
cross for fly eye color
produced a 3 : 1
phenotypic ratio of
red to white eyes in
the F2 generation.
However, none of the
flies with white eyes
were female.